1. Field of the Invention
The present invention relates to a process for preparing trimethylolpropane having a low APHA (American Public Health Association) colour number.
2. Brief Description of the Prior Art
Trimethylolpropane, referred to hereinbelow as TMP, is a trihydric alcohol of great industrial significance which finds use, for example, in the field of coating resin, powder coating, foam and polyester production.
Customarily, TMP is prepared from n-butyraldehyde and formaldehyde. Base-catalysed aldol reactions initially generate 2,2-dimethylolbutyraldehyde in the 1st reaction step from n-butyraldehyde and two equivalents of formaldehyde. 2,2-Dimethylolbutyraldehyde may then be converted in the 2nd reaction step, for example, by reaction with further formaldehyde and base in a Cannizzaro reaction to TMP-containing reaction mixtures in which the formate of the base used also occurs as a further reaction product.
The reaction steps 1 and 2 may either be carried out separately or in one working step. The bases used both for the base-catalysed reaction step 1 and also for the reaction step 2 which is stoichiometric in relation to the base quantity may optionally each independently be, for example, alkali metal or alkaline earth metal hydroxides or carbonates, or tertiary amines.
When, for example, a tertiary amine is used for the separate 2nd reaction step or the reaction steps 1 and 2 carried out in a single working step, this is known as the organic Cannizzaro process. When inorganic bases, for example alkali metal or alkaline earth metal hydroxides or carbonates, are used, this is accordingly known as an inorganic Cannizzaro process. The different physical and chemical properties of the ammonium formates or inorganic formates occurring as the further reaction products require very different work-up methods for the TMP-containing reaction mixtures.
The inorganic Cannizzaro process has the advantage that the TMP occurs in good purity and when, for example, calcium hydroxide is used as the base, the by-product is calcium formate which in turn may be used, for example, as an additive for animal nutrition, in the building materials industry, as an assistant in the leather industry, as an assistant in producing high-gloss papers, for treating scrubbing water and in smoke desulphurization.
The TMP-containing reaction mixtures obtainable by the Cannizzaro process generally have very strong coloration which is caused by impurities. This coloration, which may customarily be evaluated by a colour number according to APHA (American Public Health Association) or Hazen, interferes in some uses, which is why the work-up customarily consists of cleaning operations, for example acid treatments, extractions and/or multistage distillations. Such multistage distillations generally require expensive, space-consuming and column arrangements which are costly and inconvenient in terms of apparatus and are accordingly quite unattractive in economic terms.
Various techniques are known for the work-up of TMP-containing reaction mixtures from inorganic Cannizzaro processes. For example, DD-P-45 078 describes a process for work-up in which TMP-containing reaction mixtures obtained from inorganic Cannizzaro processes are admixed with secondary cycloaliphatic alcohols which form azeotropes with water. Then water is azeotropically distilled off together with this alcohol, and the precipitated alkali metal or alkaline earth metal formates are removed by filtration. After distilling off the excess alcohol, the crude TMP obtained is distilled for further purification.
DD Patent 287 251 discloses a process for removing high boilers from TMP. Examples of high boilers include subsequent reaction products of TMP, in particular formals, which have a higher boiling point than TMP and accordingly accumulate in the distillation residue when crude TMP is vacuum-distilled. In the process described, the addition of from 0.02 to 0.05 kg of acid/kg at least partially converts many of the high boilers back to TMP, which is intended to lead to a yield increase.
GB 1 290 036 also describes a process for decomposing high boilers in TMP-containing reaction mixtures which have been obtained by the inorganic Cannizzaro process. This involves adding cationic exchange resins and heating to from 50° C. to 140° C. to convert any formals present, which have a similar boiling point to TMP and tend to decompose at the boiling point of TMP, to products having other boiling points which can be easily distilled off.
U.S. Pat. No. 3,097,245 describes a process for preparing TMP having an APHA colour number of from 50 to 200. This colour number is achieved by limiting the reaction time to less than 7 hours, acidifying the reaction mixture to a pH of less than 7 and limiting the concentrations of the starting compounds to from 5 to 20% by weight. The reaction is also followed by treatment of the solution obtained with cationic exchange resins and strongly basic quaternary ammonium anion exchangers.
Common to all the processes mentioned is that chemical treatment methods have to be carried out which worsen both the eco-balance and the economic preparability of the product or entail considerable apparatus, and accordingly financial, costs and inconvenience to generate products having an acceptable colour number.
There is accordingly a need to provide an efficient process which makes it possible to obtain pure TMP in high yield and having a very low APHA colour number, i.e. of 20 or less, from the TMP-containing reaction mixtures prepared by the inorganic Cannizzaro process.